<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        #box {
            width: 200px;
            height: 200px;
            background: red;
            color: #fff;
        }
    </style>
</head>
<body>
    <div id="box" style="width:200px;">这是文字</div>
    <script>
        var box = document.getElementById('box');
        console.log(box.style.width);
    </script>
</body>
</html>